8.4 Liquid Oxygen---Paramagnetism and Color

Place a horseshoe magnet on an overhead projector or table and pour some liquid N2
between the poles.

Pour liquid O2 between the poles of the magnet.

Observations:

In the flask, the liquid oxygen is blue. The magnet does not
retain any of the liquid nitrogen, but liquid oxygen collects between the magnet's poles.

Explanation:

The oxygen is blue and paramagnetic (i.e. attracted to a magnet) for
the same reason: the two unpaired electrons in its outermost orbital. The electrons
both create a magnetic assymetry in the oxygen molecules and absorb light in the red
portion of the visible spectrum. This light absorbtion is a two-molecule/one-photon
transition with a wavelength of 630 nm. The entire equation to explain the oxygen's
color is:

2 O2(3Sg)
+ hv -> 2 O2(1Dg)

This is the opposite of the transition that gives rise to the red
glow in the singlet oxygen chemiluminescence demonstration (see
demonstration 5.11). A single photon carries enough energy to excite two O2
molecules simultaneously. This transition is not observed in small amounts of
oxygen gas at low pressures due to the very low probability of this three-body process.
In the liquid, however, this transition is rather common because, as a general
rule, the volume of a gas decreases by a factor of about 800 when it forms a liquid.